Battery packs, each including a plurality of batteries housed in a case so as to output a predetermined voltage and have a predetermined capacity, are widely used as power sources for various devices, vehicles, and other kinds of equipment. In particular, people have recently adopted more and more often techniques for connecting together general-purpose batteries in parallel/series to turn batteries assemblies thus formed into battery modules, each of which outputs a predetermined voltage and has a predetermined capacity, and making various combinations of such battery modules to enable their application in a wide variety of equipment (see, for example, Patent Document 1). This modularization technique reduces the size and weight of the battery modules themselves by enhancing the performance of the batteries to be packed together in the battery modules. Thus, this modularization technique provides various advantages. For example, this technique allows for assembling the battery packs more efficiently and mounting the battery packs more flexibly in a limited space such as a predetermined space in an automobile.